The field of the invention is nuclear magnetic resonance imaging (MRI) methods and systems. More particularly, the invention relates to steady state free precession (SSFP) methods for acquiring MRI data and suppressing fat signal in reconstructed images.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, Mz, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated, this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx Gy and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
Most MRI scans currently used to produce medical images require many minutes to acquire the necessary data. The reduction of this scan time is an important consideration, since reduced scan time increases patient throughput, improves patient comfort, and improves image quality by reducing motion artifacts. There is a class of pulse sequences which have a very short repetition time (TR) and result in complete scans which can be conducted in seconds rather than minutes. Whereas the more conventional pulse sequences have repetition times TR which are much greater than the spin-spin relaxation constant T2 so that the transverse magnetization has time to relax between the phase coherent excitation pulses in successive sequences, the fast pulse sequences have a repetition time TR which is less than T2 and which drives the transverse magnetization into a steady-state of equilibrium. Such techniques are referred to as steady-state free precession (SSFP) techniques.
With the recent introduction of high performance gradient systems on commercially available MRI systems these SSFP imaging pulse sequences have received more attention. Not only do they significantly shorten scan time, but they also have relatively high SNR while providing T2-like contrast based on the T2/T1 ratio of tissues.
Two major problems are associated with the SSFP acquisition method. First, the images produced have undesirably bright lipid signals due to the high T2/T1 ratio of fat spins. The bright signal complicates clinical interpretation and obscures nearby tissues of greater clinical significance. Second, when using SSFP pulse sequences signal dropout and banding artifacts can appear in regions of Bo field inhomogeneity. To reduce banding artifacts and maximize signal-to-noise (SNR) efficiency, an extremely short repetition time (“TR”) is usually desired.
Two methods to suppress fat in SSFP images are described in U.S. Pat. No. 6,307,368. In the Fluctuating Equilibrium MR (FEMR) method, RF phase cycling creates transverse magnetization that fluctuates between water and fat signal on alternating pulse sequences. The second method, Linear Combination SSFP (LCSSFP), acquires two image datasets with SSFP pulse sequences using different RF phase cycles and then linearly combines the datasets during the image reconstruction. With this approach, image data sets can be combined differently to create both fat and water images without a loss in SNR efficiency.
To operate properly the FEMR and LCSSFP fat suppression methods require the use of a SSFP pulse sequence having a very short repetition period(TR). Both FEMR and LCSSFP work best when a 180° phase shift occurs between fat and water spins during each TR interval. The ideal repetition time for perfect fat water separation at 1.5T, therefore, is approximately 2.2 ms. However, obtaining such a short TR is difficult without sacrificing readout resolution, which limits the applicability of the method.
Recently, a dual echo implementation of a 3D projection reconstruction pulse sequence was described by A. Lu, et al., Proc. 11th ISMRM, 320 (2003). In this pulse sequence a first echo is acquired during the playout of the dephasing lobe of the readout gradients to sample one radial line in k-space, and a second echo is acquired during the playout of the rephasing lobe of the readout gradients to sample an adjacent radial line in k-space. After one complete acquisition of k-space the process is repeated to acquire a second k-space data set with different RF phasing such that the two k-space data sets c combined to suppress either fat or water.